you probably either want to choose HEAD or netbsd-6. Once in there, choose a branch by the date and then choose iso, which is easy to overlook in there among all the architectures. (NOTE: If you ever want source, there is a directory there for it too.) Once you're in iso/, you can choose the one for your desired arch, e.g., NetBSD-6.0_BETA-i386.iso if you took the netbsd-6 turn at the first corner.